1. Field of the Invention
The present invention relates to a recording/reproduction method and a recording/reproduction apparatus.
2. Description of the Related Art
Phase change optical discs have been known as disk-like recording media onto which data can be recorded with a high density.
The phase change optical disc has a recording film for recording data.
Data is recorded onto a phase change optical disc in the following manner. A rotating phase change optical disc is irradiated with focused laser light, so that the recording film is heated to fuse. In this case, the physical property of the recording film is changed. A temperature which the heated recording film reaches and the cooling process of the recording film from the temperature vary depending on the power of laser light, so that data can be recorded onto a phase change optical disc.
More specifically, when the intensity of laser light is high, the recording film rapidly cools from a higher temperature state, so that the recording film becomes amorphous. When the intensity of laser light is relatively low, the recording film gradually cools from a lower temperature state, so that the recording film becomes crystalline.
In general, an amorphous portion of the recording film is called a mark, while a crystalline portion of the recording film is called a space. By forming marks and spaces in the recording film, binary information can be stored.
In phase change optical discs, past data can be erased and new data can be recorded, with a singly type of laser light, i.e., direct over-write can be performed.
Data is reproduced from a phase change optical disc in the following manner. A phase change optical disc is irradiated with weak laser light which cannot elicit a phase change in the recording film. Light reflected from the phase change optical disc is detected. Amorphous marks have low reflectivity, while crystalline spaces have high reflectivity. Therefore, by detecting the difference between the amounts of light reflected from marks and spaces, data can be reproduced from a phase change optical disc.
When a long mark is recorded onto a phase change optical disc by irradiating it with laser having a predetermined power, the radical thickness of the mark is gradually increased toward the trailing edge thereof due to the thermal storage effect of the recording film. In this case, a mark may be incompletely erased in direct over-write, or signal crosstalk may occur between tracks when reproducing data from a phase change optical disc, so that the quality of the phase change optical disc is largely impaired.
As described above, the reflectivity of a mark is lower than the reflectivity of a space in a phase change optical disc. This means that the heat absorption of a mark is higher than the heat absorption of a space. The heat of fusion required for fusing the amorphous portion is different from the heat of fusion required for fusing the crystalline portion.
Therefore, when a new mark is formed by irradiating an existing mark and space with laser light having the same power in direct over-write, the amount of heat absorption and the achieved temperature differs between the mark and the space. Thus, the edge of the new mark varies depending on the previous recorded state. Particularly in conventional recording operation, the amount of light for irradiating the second half of a mark is reduced. A problem arises such that the position of the edge of the second half of a mark varies significantly.
The length of a mark and a space is considered to be reduced in order to increase data recording density in the future. Particularly when the length of a space is short, heat which has been used to form the trailing edge portion of a mark is transferred to a space adjacent to the mark and then to the leading edge portion of the next mark. In this case, an increase in the temperature of the next mark is affected, or conversely, heat which is used to form the leading edge portion of a mark may have an influence on the cooling process for the trailing edge portion of the previous mark, i.e., thermal interference may occur. If thermal interference occurs in conventional recording operation, the edge positions of marks fluctuate, leading to an increase in an error rate in reproducing a phase change optical disc.
Japanese Patent No. 2679596 discloses a technique for solving the above-described problems (see FIG. 2). Here, a signal corresponding to a mark in the Pulse Width Modulation (PWM) recording format is divided into a leading edge portion having a predetermined width, a burst-like intermediate portion, and a trailing edge portion having a predetermined width. The signal is recorded by switching two-level laser output at high rate. To produce the intermediate portion of a long mark, a burst-like laser current is driven so that a disc is irradiated with light having a minimum power. As a result, the intermediate portion of the mark is not expanded and has a substantially constant width. A sufficient amount of laser light having a constant width is used in order to produce the leading edge portion and trailing edge portion of a mark. Therefore, jitter is not increased at the edge portion of the formed mark in direct over-write. In addition, when it is detected that a mark has a small length and spaces before and after the mark also have a small length, a mark is formed such that the positions of the leading edge portion and trailing edge portion of the mark are different from when the mark and the spaces have a long length. Thereby, thermal interference or peak shift due to reproduced frequency characteristics are compensated for in recording.
Japanese Laid-Open Publication No. 2000-231719 discloses a recording method in which a particular repetition pattern is recorded; the optimum amounts of movement of a first pulse and a last pulse are determined depending on data pattern; movement amount information indicating both or either of the determined movement amounts of the first and last pulses is recorded onto a recording medium during manufacture (see FIGS. 2 and 11-14). When the user records data onto the recording medium, the movement amount information is read out from the recording medium and recording is performed using the movement amount information. Thereby, learning for recording a mark on a correct position can be omitted or the time required for learning can be reduced, while the accuracy of positioning marks can be improved. Thus, jitter is reduced.
However, it is believed that data density is increased more and more in the future. In this case, the techniques described in the above publications each have a problem.
In the recording operation described in Japanese Patent No. 2679596, the length of a mark and the lengths of spaces before and after the mark are detected; when the length of the mark is small and the lengths of spaces before and after the mark are small, the leading edge portion and trailing edge portion of the mark is formed so that they are different from the leading edge portion and trailing edge portion of a mark formed when the length of the mark and the lengths of spaces are long; thereby, thermal interference or peak shift due to reproduced frequency characteristics are compensated for in recording. Whereas the recording operation has high performance, the scale of an apparatus for achieving the recording operation is increased, leading to an increase in cost.
In the recording operation described in Japanese Laid-Open Publication No. 2000-231719, a particular repetition pattern is recorded; the optimum amounts of movement of a first pulse and a last pulse are determined depending on data pattern; movement amount information indicating both or either of the determined movement amounts of the first and last pulses is recorded onto a recording medium during manufacture; when the user records data onto the recording medium, the movement amount information is read out from the recording medium and a mark is recorded at a correct position. In the recording operation, the particular repetition pattern is recorded. Therefore, the scale of an apparatus for achieving the recording operation is increased, leading to an increase in cost.
Japanese Laid-Open Publication No. 2000-231719 also describes a recording operation in which a first pulse and a last pulse are moved by an optimum movement amount depending on a disc. However, it is not described that the width of a pulse (or multi-pulse) between the first and last pulses is changed or the width of a cooling pulse is changed in order to optimize recording/reproduction characteristics.